The Video Drift Method to Measure Double Stars

Richard Nugent 33rd IOTA Annual Meeting

Las Vegas, Nevada October 17, 2015

WHY MEASURE DOUBLE STARS ?

Position angles (θ) and separations (ρ) allow computation of orbits

Orbital periods Newton’s version Kepler’s 3rd law Actual separations/distances and stellar masses

Distance absolute luminosity

H-R Diagram the basis for the distance scale of

the Universe

METHODS TO MEASURE DOUBLE STARS

Micrometer/Visual

drift

Fig. 1 Fig. 2

Accuracy ~ ±1°, ± 1″

Photographic/CCD Camera Method

Take image Measure positions of double

star and reference stars Perform astrometric (plate)

reduction Requires star catalogue

With (RA, DEC) of components, compute position angle (PA) and separation (Sep)

Accuracy ~ ± 0.3°, ± 0.3″

(combining multiple exposures)

Mann Measuring Engine x

y

Video Camera Methods

Individual frames are chosen and stacked for a relative astrometric reduction

East-West direction is derived from widely spaced frames

Calibration doubles required

• Video record double stars drifting across the FOV-Motor drive off

• Video is analyzed with Limovie* - output is brightness data and standard (x, y) coordinates of components for each video frame

• (x, y) coordinates are input into Excel program VidPro to calculate PA, SEP, scale factor and other statistical quantities

Nugent-Iverson Video Drift Method

*Limovie written in 2005 by Kazuhisa Miyashita, Japan

Dat

a to

be

sent

to

CSV

File

Start End

Limovie CSV file

Limovie CSV file

Watec 902H camera

Collins Image Intensifier

Meade 14“ LX-200

VidPro – VIdeo Drift PRogram reductiOn

VidPro – VIdeo Drift PRogram reductiOn

= (x,y)

= photometry

Limovie’s Aperture Rings

• Brightest pixel in red aperture is identified

• All pixels with brightness of at least 50% of max value are assumed to be part of the star image

• (x,y) = center of gravity of the top 50% pixels is recorded as the position of the star

Limovie’s Centroid Determination

With Limovie’s (x,y) position output for each star, separation, position angle, scale factor are computed automatically as follows:

ρ = { (xp – xs)² + (yp – ys)² } x scale factor

θ = tan -1 (xp– xs)(yp– ys)

drift-time = seconds from GPS time of drift endpoints or drift-time = total frames

cos (xB – xE)² + (yB – yE)²

(drift-time) 15.041068scale factor =

frame rate

Correction to Position Angle

Camera’s video chip orientation

actual east-west driftdrift angle

Drift angle calculation - method 1

= cos

Drift angle calculation - method 1 triangle solution

-1 ABAC

Drift angle calculation - method 2 least squares

y = mx + b

drift angle = arctan (m)

Accuracy ~ ± 0.02°

RESULTS COMPARED TO

Washington Double Star Catalog (WDS)

° "

STATISICAL RESULTS TO DATE

For 1,133 doubles measured:

PA, average σ = 1.14°

Sep, average σ = 0.37″

Closest separation = 3.7″

WDS 20391-0942 J1400AB, Sep = 3.7″

How faint can we go ?• Nugent’s system:

14″ Meade SCT Watec 902H Ultimate

Collins I3 intensifier

Elevation: 1,705 m

Mag limit: routinely reach +15 in the dry dark skies of west Texas (5 miles from McDonald Observatory)

• Iverson’s system:

14″ Meade SCT Stella Cam 3 (Watec 120N)

Elevation: 92 m

Mag limit: +12 in the humid average skies in east Texas

Integrating Video Cameras Integrating video cameras add (integrate) a

predetermined number of frames and then continuously output that image until the next image is available.

In simplistic terms, each successive increase in

the integration level doubles the exposure time.

Using a Stella Cam 3, Iverson can see deeper than magnitude +12, but there are serious problems measuring faint double stars.

+ + + =

The Problem At integration levels greater than 4 frames

(0.132 sec.) the target stars are elongated and jump during drift

Limovie is unable to follow these jumps and therefore it has trouble tracking the drifting stars.

Limovie is also unable to accurately resolve the (x,y) coordinates for the primary and secondary stars.

A Drift Example with 3-sec Frame Integration

The Modified Video Drift Method

• Using the Modified Video Drift method Iverson has measured double stars down to magnitude +16.9.

• In April 2015 we published a short paper describing the method (JDSO, vol. 11, No. 1)

Method consists of 2 Phases:Tracking a) Use integrating video to acquire faint double star.

b) Record video for 1-2 minutes – motor drive running

c) Stars are stationary in FOV

Drifting a) Turn off integration and motor drive (video still

b) Video recording rate is back at 30 fps (25 fps PAL)

c) Slew telescope east or west until a bright star is visible at same (or close to) declination as target double

d) Continuing, record 3 or 4 drifts as in the regular video

recording)

drift method

phase

phase

• Tracking (integrating) the double stars allows the appropriate level of integration to see the double star components

• Stars stationary – no skipping/jumping, and Limovie can thus measure their (x, y) coordinates

• Drifting – allows drift angle and scale factor to be derived from the same video

Modified Drift Video Example

Reducing the video data: • Drift phase - place both Limovie aperture rings over the single bright star Use CSV file into VidPro - computes plate scale/drift angle

• When you do this, the position angle/separation results reported by VidPro are meaningless, but the plate scale and drift angle are valid

• Tracking phase – Measure with Limovie, insert CSV file into Vidpro. Manually insert drift angle/scale factor into 2nd version of VidPro,

Manually insert plate scale here

Manually insert drift angle here

VidPro Modifications

Advantages of this Drift Method• Only the component double stars need be visible and measured, no other stars needed (Modified Drift method requires 3rd brighter star)

Advantages of this Drift Method• Only the component double stars need be visible and measured, no other stars needed (Modified Drift method requires 3rd brighter star)

• Each video frame provides PA, SEP which are all averaged. All frames are used – none are discarded

Advantages of this Drift Method• Only the component double stars need be visible and measured, no other stars needed (Modified Drift method requires 3rd brighter star)

• Each video frame provides PA, SEP which are all averaged. All frames are used – none are discarded

• Scale factors/drift angles computed automatically to ±0.03″/ ±0.02°

Advantages of this Drift Method• Only the component double stars need be visible and measured, no other stars needed (Modified Drift method requires 3rd brighter star)

• Each video frame provides PA, SEP which are all averaged. All frames are used – none are discarded

• Scale factors/drift angles computed automatically to ±0.03″, 0.02°

• No dark frames, flat/bias frames, no shielding or cooling of video camera

Advantages of this Drift Method• Only the component double stars need be visible and measured, no other stars needed (Modified Drift method requires 3rd brighter star)

• Each video frame provides PA, SEP which are all averaged. All frames are used – none are discarded

• Scale factors/drift angles computed automatically to ±0.03″, 0.02°

• No dark frames, flat/bias frames, no shielding or cooling of video camera

• Each drift is self calibrating: hardware can be removed and re-installed on telescope without having to worry about re-calibrating

Advantages of this Drift Method• Only the component double stars need be visible and measured, no other stars needed (Modified Drift method requires 3rd brighter star)

• Each video frame provides PA, SEP which are all averaged. All frames are used – none are discarded

• Scale factors/drift angles computed automatically to ±0.03″, 0.02°

• No dark frames, flat/bias frames, no shielding or cooling of video camera

• Each drift is self calibrating: hardware can be removed and re-installed on telescope without having to worry about re-calibrating

• Standard deviations reflect realistic errors in measurements

Advantages of this Drift Method• Only the component double stars need be visible and measured, no other stars needed (Modified Drift method requires 3rd brighter star)

• Each video frame provides PA, SEP which are all averaged. All frames are used – none are discarded

• Scale factors/drift angles computed automatically to ±0.03″, 0.02°

• No dark frames, flat/bias frames, no shielding or cooling of video camera

• Each drift is self calibrating: hardware can be removed and re-installed on telescope without having to worry about re-calibrating

• Standard deviations reflect realistic errors in measurements

• No star catalogues, no calibration doubles, no precession needed – PA, SEP are for equator and epoch of date – what you see is what you get

Advantages of this Drift Method• Only the component double stars need be visible and measured, no other stars needed (Modified Drift method requires 3rd brighter star)

• Each video frame provides PA, SEP which are all averaged. All frames are used – none are discarded

• Scale factors/drift angles computed automatically to ±0.03″, 0.02°

• No dark frames, flat/bias frames, no shielding or cooling of video camera

• Each drift is self calibrating: hardware can be removed and re-installed on telescope without having to worry about re-calibrating

• Standard deviations reflect realistic errors in measurements

• No star catalogues, no calibration doubles, no precession needed – PA, SEP are for equator and epoch of date – what you see is what you get

• Large # of (x,y) data pairs are unprecedented in the data analysis compared to any other double star measurement technique

LX-200 Users – No need to turn Scope off for Drift

Conclusions

• Our drift video method is an alternative method to measure double stars

• This drift method produces results for PA and separations with high systematic accuracy

• This technique uses a feature of Limovie that was previously overlooked

•Web page for VidPro download:

http://www.poyntsource.com/Richard/double_stars_video.htm